J.L. Casamor

Seafloor evidence of a subglacial sedimentary system off the northern Antarctic Peninsula

Swath-bathymetry data and high-resolution seismic reflection profiles allow us to portray a subglacial sedimentary system off the northern tip of the Antarctic Peninsula, in the Central Bransfield Basin, during the Last Glacial Maximum with unprecedented detail. Postglacial reworking and sedimentation are weak enough for the subglacial morphology of the Last Glacial Maximum to be preserved on the present seafloor. The studied sedimentary system extends 250 km, from ∼1000 m above sea level to ∼2000 m water depth. The data set supports a model for subglacial sedimentary systems that consists of: (1) an upper ice catchment or erosional zone on the innermost continental shelf, extending onshore; (2) a transitional erosional-depositional zone on the inner shelf with drumlinized seafloor; (3) a depositional outer shelf zone with mega-scale bundle glacial lineations; and (4) a debris apron on the continental slope and base of slope formed under floating ice shelves with debris delivery linked to grounding lines along the shelf break.

The Balearic Promontory geomorphology (western Mediterranean): morphostructure and active processes

In this paper, a detailed study of the submarine geomorphology surrounding the Balearic Promontory (western Mediterranean), a northeast prolongation of the Neogene Betic Range in southern Spain, is presented from a series of high-resolution tools including swath bathymetry and seismic reflection profiling. The study identifies the main features of the continental shelf, slope and basins surrounding the Balearic Islands. We show a variety of seafloor relief that owes its origin to several geologic processes, which ultimately control the transport of sediment from the shallower areas to the deep basin. The most important processes are erosion of the shelf and upper slope (terraces associated with different Quaternary sea-level stands and canyons), transport and sediment deposition in the lower slope and base-of-slope by turbidity currents, volcanism and instability processes (landslides scarps and debris lobes). The swath data show that tectonics plays an important role in shaping the submarine slopes of Eivissa and Formentera, the two southernmost islands, as well as its interplay with sedimentary processes, especially mass wasting. Finally, several areas show evidence of pockmarks, which indicate that fluid migration take place in the sediments, probably conditioning several other processes such as mass wasting.

Sediment undulations on the Llobregat prodelta: Signs of early slope instability or sedimentary bedforms?

A field of sediment undulations has been mapped by means of high resolution multibeam bathymetry and seismic reflection profiles in the Llobregat River prodelta, off the city of Barcelona, Catalonia, Spain. Similar features had previously been recognized in other prodelta environments and interpreted either as downslope sediment deformation or sedimentary structures induced by bottom currents or hyperpycnal flows. Since the study area is undergoing significant offshore development, proper interpretation of such sediment undulations is needed for a correct risk assessment. The occurrence of the sedi- ment undulations is restricted to the prodelta front on slope gradients between 3 and 0.2o. The undulations have developed at the edge and atop an area of gas bearing sediments within the Late-Holocene high-stand mud wedge. An evaluation is made of the character- istics of the sediment undulations in order to determine the most likely process for the origin of these structures. Amongst these characteristics are the continuity of the reflec- tions and lack of diffractions in between different undulations, their size distribution (large to small) both from shallow to deep and with depth in section, the asymmetry (de- creasing from proximal to distal), the crest to trough vertical distance on the landward side of the undulations (up to 0.5 m), and the lack of features that could indicate a pro- gressive movement such as growth structures and drag folds. These characteristics indi- cate that the sediment undulations on the Llobregat River prodelta do not result from sediment deformation, but rather from the interaction of bottom currents generated by hyperpycnal flows from the Llobregat River with regional sea water circulation. Their identification as sediment waves implies that such features do not pose a major hazard for further offshore development.

Deep-water coral occurrences in the Strait of Gibraltar

This study reports for the first time on the occurrences of deep-water coral species in the Spanish territorial waters of the Strait of Gibraltar. Based on an extensive dataset of 334 grab samples, 16 species of calcareous corals have been identified in water depths between 13–443 m. Scleractinian corals form the dominant benthic community between 140–330 m water depth. The corals appear on the seabed both as solitary individuals and as patches on small biological topographic build-ups. The most common coral species Lophelia pertusa and Madrepora oculata are associated with coarse-grained calcareous sediments and mound structures. In the shallowest part of the study area (<150 m) algae and bryozoans are dominant and only a few coral species are observed. This zonation and the occurrence of the azooxanthellate corals in the Strait of Gibraltar relates to light availability and perhaps also to the complex interaction between the outflow of Mediterranean water and surficial inflow of Atlantic water into the Mediterranean Sea.

The BIG’95 Submarine Landslide–Generated Tsunami: A Numerical Simulation

This article presents a reasonable present-day, sea-level highstand numerical simulation and scenario for a potential tsunami generated by a landslide with the characteristics of the BIG’95 debris flow, which occurred on the Ebro margin in the western Mediterranean Sea in prehistoric times (11,500 cal yr BP). The submarine landslide deposit covers an area of 2200 km2 of the slope and base of slope (200–1800-m water depth), involving a volume of 26 km3. A leapfrog finite difference model, COMCOT (Cornell multigrid coupled tsunami model), is used to simulate the propagation of the debris-flow-generated tsunami and its associated impact on the nearby Balearic Islands and Iberian coastlines. As a requisite of the model, reconstruction of the bathymetry before the landslide occurrence and seafloor variation during landsliding have been developed based on the conceptual and numerical model of Lastras et al. (2005). We have also taken into account all available multibeam bathymetry of the area and high-resolution seismic profiles of the debris flow deposit. The results of the numerical simulation are displayed using plots of snapshots at consecutive times, marigrams of synthetic stations, maximum amplitude plots, and spectral analyses. The obtained outputs show that the nearest shoreline, the Iberian coast, would not be the first one hit by the tsunami. The eastward, outgoing wave would arrive at Eivissa Island 18 min after the triggering of the slide and at Mallorca Island 9 min later, whereas the westward-spreading wave would hit the Iberian Peninsula 54 min after the slide was triggered. This noticeable delay in the arrival times at the peninsula is produced by the asymmetric bathymetry of the Catalano-Balearic Sea and the shoaling effect due to the presence of the wide Ebro continental shelf, which in addition significantly amplifies the tsunami wave (>9 m). The wave amplitudes attain 8 m in Eivissa, and waves up to 3 m high would arrive to Palma Bay. Resonance effects produced in the narrow Santa Ponça Bay in Mallorca Island could produce waves up to 9 m high. A similar event occurring today would have catastrophic consequences, especially in summer when human use of these tourist coasts increases significantly.

The Equatorial Atlantic Mid-Ocean Channel: An Ultra High-Resolution Image of Its Burial History Based on TOPAS Profiles

Multibeam bathymetric and ultra high-resolution seismic data reveal that the distal course of the Equatorial Atlantic Mid-Ocean Channel (EAMOC) extends further east and south than was previously known, and is controlled by the presence of morphologic highs related to the Fernando de Noronha Fracture Zone. Distal course of the EAMOC is buried by sediments, and does not have bathymetric expression on the seafloor. The channel fill consists of three seismic sequences, suggesting that the recent geological evolution of the channel is composed of successive phases of decreasing sedimentary activity that finally resulted in its complete burial. Tectonic and volcanic activity related to the Fernando de Noronha Fracture Zone and Ridge, together with the effect of strong pulses of the Antarctic bottom water current during the upper Pliocene are suggested to have contributed to the progressive burial and the final abandonment of the EAMOC.

The seafloor imprint of the Gerlache–Boyd Ice Stream (65–62° S), northern Antarctic Peninsula

The northern Antarctic Peninsula (NAP) forms a narrow stretch of land that extends to a relatively low latitude (63° S) and is subject to a humid, maritime-influenced climate, especially on its western side. During the Last Glacial Maximum (LGM), the NAP was covered by the northern part of the Antarctic Peninsula Ice Sheet (APIS) (Lavoie et al. 2015). The APIS fed ice streams flowing on both sides of the NAP, including the Gerlache–Boyd Ice Stream (GBIS) (Canals et al. 2000). Fast-flowing ice streams are the most dynamic components of ice sheets and largely determine ice-sheet mass loss and stability (Bentley 1987; Bamber et al. 2000). They transport large volumes of sediments both subglacially and englacially, and shape a variety of landforms both on land and the seafloor (e.g. Dowdeswell & Elverhoi 2002). During the LGM and throughout deglaciation, the role of ice streams was particularly significant. We describe the landforms associated with the GBIS based on a comprehensive compilation of multibeam bathymetry. The set of landforms and deposits left by the GBIS ranges in age from LGM to present, and illustrates both the products of ice-stream dynamics and how post-glacial processes can mask this glacial imprint. Our aim is to understand the dynamics of the GBIS and its temporal evolution since the LGM. In addition, the role of underlying geological control on the overall physiography of the GBIS is assessed. The study area extends 365 km from the southern end of Gerlache Strait (GS) to the South Shetland deep-sea trench and is fed by a glacierized catchment of about 23 000 km2 (Fig. 1a) (Canals et al. 2000). The height difference from the ice divide to the deep-sea trench is c. 6700 m. The GBIS system was the main drainage pathway of this catchment (O Cofaigh et …

Tsunami Resonance in Palma Bay and Harbor, Majorca Island, as Induced by the 2003 Western Mediterranean Earthquake

The tsunami induced by the May 21, 2003, Algerian Boumerdès-Zemmouri earthquake (moment magnitude Mw = 6.9) propagated across the western Mediterranean Basin, thereby causing material damages in some harbors and coastal areas. This was the case in the Balearic Islands and particularly the Palma harbor. Attempts to simulate the 2003 tsunami event found discrepancies between tsunami arrival times and wave amplitude when comparing tide gauge records with results from numerical models. To date, all published model results of the amplitude of the tsunami are underestimations, attributed to numerical limitations due to the lack of high-resolution bathymetry and poor definition of harbor geometry. Other views suggest the inappropriateness of the available seismic source parameterizations and the possible occurrence of submarine landsliding triggered by the earthquake that has not been included in the numerical simulations. In this article we present the results of a numerical study aimed at better understanding the response of the interacting Palma bay and harbor under the impact of the 2003 western Mediterranean tsunami. The transference of the tsunami energy from the generation area to the continental shelf, the bay, and the harbor has been studied and compared with the natural oscillation modes of the bay and harbor water bodies. Our work includes a sensitivity analysis of the source parameterization and the bathymetry grid size for the bay and harbor as a way to explain the discrepancies between simulations and observations. The Palma harbor tide gauge shows that energy from the tsunami concentrated mainly in periods that fitted to the natural modes of oscillations of the bay. Therefore, the significant wave amplification observed inside the harbor, mainly in its northern basin, was generated by a resonance effect induced by Palma bay. The improvement of the bathymetry grid resolution in the bay and harbor domains and the inclusion in the simulations of the exact harbor geometry and internal configuration result in a slight wave-high increment that is much below the wave height recorded in the tide gauge. Our results strongly point to a necessary revision of the tsunami seismic source parameters.

Gebra Slide: glacial and tectonic controls on recurrent submarine landsliding off the northern tip of the Antarctic Peninsula

Late Quaternary–Holocene submarine landslides occurring in high-latitude, glacial-influenced continental margins have been increasingly investigated during the last three decades. Both relatively small and large landslides have been identified in these settings (e.g. Canals et al. 2004; Vanneste et al. 2006). However, much more evidence on submarine instabilities is available from the Arctic than the Antarctic. Bransfield Basin (BB) is a 450 km long, SW–NE-orientated, marginal rift basin opening between the South Shetland archipelago and the northern tip of the Antarctic Peninsula (AP) (Fig. 1a, b). The middle part of BB is occupied by the 215 km long and 120 km wide Central Bransfield Basin (CBB), bounded at its ends by transverse ridges forming the volcanoes of Deception (not shown) and Bridgeman islands. CBB is asymmetrical, with a narrow and steep South Shetland margin and a broader and gentler AP margin, where the continental shelf is about 70 km wide (Fig. 1a). Fig. 1. ( a ) Shaded-relief bathymetric image of the Central Bransfield Basin. Acquisition system Simrad EM12-S. Frequency 12.5 kHz. Grid-cell size 200 m (colour image) and GEBCO_08 Grid (grey image). Land area is from the Landsat Image Mosaic of Antarctica (US Geological Survey). ( b ) Location of study area (red box; map from IBCSO v. 1.0). SSI and BB refer to South Shetland …

Cross-shelf troughs in Central Bransfield Basin, Antarctic Peninsula

During the Last Glacial Maximum (LGM), the ice masses covering the western side of the northernmost Antarctic Peninsula (NAP) drained into Bransfield Basin (BB) (Banfield & Anderson 1995). A drainage-basin area of about 60 000 km2 generated an ice flux that entrained significant volumes of englacial and subglacial debris that were subsequently delivered into BB. Multibeam-bathymetric data show a number of deeply sculpted, sub-parallel glacial cross-shelf troughs and associated landforms within the framework of an entire glacial depositional system (Canals et al. 2002; Garcia et al. 2011). Bransfield Basin is a 35 000 km2, SW–NE-orientated marginal basin situated between the South Shetland Islands (SSI) and the NAP and adjacent islands (Fig. 1a). BB consists of three sub-basins, the main one being the 1950 m deep Central Bransfield Basin (CBB) which is 215 km long and 120 km wide. The NAP margin of the CBB hosts several cross-shelf troughs opening into a sedimentary outer shelf (Fig. 1b) including Mott Snowfield, Laclavere and Lafond troughs (Canals et al. 2002). Fig. 1. ( a ) Location of study area (red box; map from IBCSO v. 1.0). SSI, South Shetland Islands; BB, Bransfield Basin. ( b ) Swath-bathymetric image of Central Bransfield Basin (CBB). Acquisition system Simrad EM12-S. Frequency 12.5 kHz. Grid-cell size 200 m (colour) and GEBCO_08 Grid (grey). Land area …